Suppose we have two operators A and B, such that they are both hermitian with eigenvalues ai and bi. We construct a new operator like this C=A +iB, where i is the imaginary unity. Operator C is observable in the sense that it is measurable. Yes, if we measure A and B (suppose for simplicity that they commute) we get that the measured value of C is a+ib. Thus we obtain an OBSERVABLE with complex eigenvalues, in contradiction with the 3rd postulates wich states that all observable quantities are associated with hermitian operators with complete bases of eigenstates, bla bla. Kinda stupid but it works.